Unidirectional fabrics are fabrics in which the warp and weft yarns form parallel layers, without the over and under crimp of a woven structure. Without such an interwoven structure, some mechanism must be provided to hold the layers of unidirectional yarn together to form the fabric. Some known mechanisms include the use of resins, polymer films bonded to the individual layers, stitching, knitted fabric layers and woven fabric layers.
When unidirectional fabric is used in the reinforcement of composites, the composites generally have a number of layers of unidirectional yarns with at least two of the layers being oriented transversely (at 90 degrees) to each other. Typical arrangements include two yarn layers, with the first layer oriented at 0 degrees to the longitudinal direction of the fabric, and the second layer oriented at 90 degrees to first layer. In other arrangements, fabric layers may be oriented at +/−45 degrees to the longitudinal direction of the fabric.
In known composites, the yarn layers may be stitched together, usually with stitch lines that are closely spaced together. The angle at which the layers of yarns are oriented with respect to each other may be varied, and the spacing of the stitching and the length of the individual stitches may also be varied.
However, there are a number of drawbacks with stitched fabric. Since the stitched yarns are woven by needles that penetrate through the structural yarn during production, considerable gaps may be formed where the stitches are provided. Furthermore, the penetration of the needles may cause severe damage to the structural yarns.
Some fabrics have been produced with and without a thermoplastic film provided between the yarn layers. With film provided between the layers, the fabric may be hot pressed such that the film can soften or melt during pressing, thus bonding with the layers and serving as a resin system in the finished composite.
Another family of unidirectional fabrics involves impregnating a unidirectional layer of filaments of high performance yarn with a thermoplastic or thermoset resin system. Two layers of the resultant prepreg can then be cross-plied together at a predetermined angle (e.g. 90 degrees) to form a single sheet of material.
Unidirectional fabric may used to overcome some of the reduced performance in structural materials caused by the crimping found in woven fabrics. A prior art woven fabric 10 is shown in FIGS. 1 and 2, and includes warp yarns 12 and weft yarns 14 that are interwoven. The warp yarns 12 and weft yarns 14 in the fabric 10 are crimped, as each yarn is bent around other yarns at crossover points or nodes to provide an interlocking or interwoven structure. As a result of the crimping, less than optimal mechanical properties may be provided in the fabric 10.
Various techniques have been used to reduce the fabric crimp in woven fabrics and to spread the crossover points apart. One way to achieve this is by weaving yarn in a more open construction, while retaining the weave pattern. To achieve the desired performance, the individual yarns in the fabric must typically be flat and spread apart to provide an open construction for the fabric. Without flat, spread yarns, the interstices or gaps between the yarns tend to become excessive. This reduces the amount of fibre in a given volume in the final composite structure and tends to result in excess resin content required to fill in the gaps, reducing the overall performance.
Improvements in yarn manufacture and weaving technology have allowed some high performance yarns to be woven with little or no twist and with a resulting flat, spread yarn orientation to address the gap issue. However, there is a limit to the openness of the weave that can be achieved with a woven fabric. As the openness increases, the fabric tends to become more of a mesh or scrim as opposed to a true fabric, and may have little or no value in structural applications. In addition, the fabric may become so flimsy that it cannot be handled or cut without distorting the orientation of the yarns and ruining the fabric, making it difficult to work with.
Accordingly, there is a need for an improved high performance fabric for use in structural applications that includes at least some of the benefits of unidirectional fabrics and woven fabrics while overcoming at least some of the above noted disadvantages associated with such fabrics.